Armed protesters who ended their 41-day standoff on Thursday at a wildlife refuge in Oregon told federal authorities they left behind booby traps but did not say whether the trip wires and other devices would trigger explosions, a law enforcement official told Reuters.

“They spoke to us about booby traps. We don’t know how sophisticated or what kind,” said the official, speaking on the condition of anonymity.

Materials to create explosives could be found on the refuge, the official said, because workers there previously performed controlled burns of the land.

The official said law enforcement would use caution when moving into the refuge in remote eastern Oregon.

“There were materials that could be used to create hazardous devices, so it’s just very prudent for us to do to that,” the official said.

The final four occupiers left the Malheur National Wildlife Refuge on Thursday and joined 12 other occupiers to face federal charges.

The takeover, which began on Jan. 2, was sparked by the return to prison of two Oregon ranchers convicted of setting fires that spread to federal property in the vicinity of the refuge.

The armed protesters, originally led by brothers Ammon and Ryan Bundy, were known to be heavily armed and resistant to the federal government.

Ammon and Ryan Bundy were arrested on Jan. 26 and have since called for protesters to leave.

Their father, Cliven Bundy, flew to Oregon on Wednesday to support the movement but was arrested at the Portland airport on charges stemming from the 2014 standoff on his Nevada ranch.

The official said law enforcement arrested Cliven Bundy at the airport because they knew he would be unarmed and the charges against him were ready.